176 HOW CROPS FEED. 
be regarded for the most part as purely physical. There 
are other results of the action of the soil on saline solu- 
tions, which, though perhaps influenced by simple physical 
action, are preponderatingly chemical in their aspect. 
These effects, which manifest themselves by chemical de- 
compositions and substitutions, will be fully discussed in 
a subsequent chapter, p. 333. 
§ 6. 
PERMEABILITY OF SOILS TO LIQUID WATER. IMBIBITION. 
CAPILLARY POWER. 
The fertility of the soil is greatly influenced by its de- 
portment toward water in the liquid state. 
A soil is permeable to water when it allows that liquid 
to soak into or run through it. To be permeable is of 
course to be porous. On the size of the pores depends its 
degree of permeability. Coarse sands, and soils which 
have few but large pores or interspaces, allow water to 
run through them readily—water percolates them. When, 
instead of running through, the water is largely absorbed 
and held by the soil, the latter is said to possess great 
capillary power ; such a soil has many and minute pores. 
The cause of capillarity is the same surface attraction 
which has been already under notice. 
When a narrow vial is partly filled with water, it will 
be seen that the liquid adheres to its sides, and if it be not 
more than one-half inch in diameter, the surface of the 
liquid will be curved or concave. In a very narrow tube 
the liquid will rise to a considerable height. In these 
cases the surface attraction of the glass for the water neu- 
tralizes or overcomes the weight of (earth’s attraction for) 
the latter. 
The pores of a sponge raise and hold water in them, in 
the same way that these narrow (capillary *) tubes sup- 
_ * From capillus, the Latin word for hair, because as fine as hair; (but a hair is 
no tube, as is often supposed.) 
